Spark gap light source for impact photoelasticity



Oct, 28, 1969 E. CHAPMAN 3,475,646

l SPARK GAP LIGHT SOURCE FOR IMPACT PHOTOELASTICITY Filed April 10, 19672 Sheets-Sheet l Get. 28, 1969 CHAPMAN 3,475,646

SPARK GAPLIGHT SOURCE FOR IMPACT PHOTOELASTICITY Filed April 10, 1967 .2Sheets-Sheet 2 United States Patent 3,475,646 SPARK GAP LIGHT SOURCE FORIMPACT PHOTOELASTICITY Everett Chapman, P.0. Box 207, West Chester, Pa.19380 Filed Apr. 10, 1967, Ser. No. 629,749 Int. Cl. H01t 7/00 US. Cl.315-59 7 Claims ABSTRACT OF THE DISCLOSURE A spark gap device for use inimpact photoelasticity which provides for high intensity, short durationillumination and thereby permits the use of impact photoelasticspecimens made of stiff plastic materials having traveling wavevelocities comparable to velocities occurring in materials subject toimpact. The device comprises a bank of capacitors coupled to a spark gapby circuit means whose time constant provides for the gap discharge toreach maximum value practically instantaneously.

This invention relates to photoelasticity and in particular relates toequipment for obtaining photoelastic images under impact conditions.

Conventional photoelastic work contemplates imposing a static load on aspecimen, recording the resulting image, and analyzing the stressdistribution. While the analysis of images attained by static loading isvitally important in stress analysis work for obtaining optimum designs,it is well known that stress distributions vary widely as between staticand impact or transient conditions. For example, it has been found thatmany parts whose stress patterns obtained under static conditionsindicate ideal design will fail under impact conditions.

Previous attempts to produce photoelastic images with a specimen loadedby impact have not accurately reproduced the conditions which prevail inmaterials subject to impact. One of the reasons for this has been thelack of a light source having an intensity and time duration compatiblewith the velocity of the traveling wave in the material. The lack of afast light source has caused the researchers to use plastic materials asspecimens having traveling wave velocities compatible with the availablelight sources. Such materials are relatively soft or rubbery and do notrespond to impact in the same manner as stiif materials. Thus materialshaving wave velocities far below the velocities which actually occur inpractice and which do not respond properly have made it impossible tofairly duplicate the actual conditions and fail to obtain the trueimpact or transient stress distribution patterns.

The present invention contemplates a light source of high intensity andof extremely short time duration which enables that use of specimensmade of stitf plastics having impact response and wave velocitiescompatible with velocities in materials subject to impact loads.According to the invention, the time of impact and the time ofappearance of the light are correlated so that the traveling wave can bestopped at the moment of impact and at finite points as it travels alongthe specimen and thereby obtain a series of images showing the wave asit is first generated and then as it spreads into the specimen.

charge current of the capacitors is extremely steep; for example, withthe device described herein I have obtained arcs or flashes existing forless than nanoseconds.

With discharge times of the order indicated, the invention contemplatesspecimens made from photoelastically sensitive plastic materials such asepoxy resins having elastic moduli from approximately 50,000 to 500,-000 p.s.i. and corresponding velocities from approximately 20,000 to62,000 inches per second. The wave velocities of plastics of this naturecorrespond to wave velocities in materials and alloys commonly used forapplications where the same are subject to impact loads.

The use of a spark gap having a very fast discharge time is especiallyadvantageous for plastics of the kind mentioned and for photographingthe transient image. The spectral distribution of a spark gap in air hasmany strong lines from about 3,500 Angstrom to 4,800 Angstrom, therebeing very strong blue lines, some yellow lines, and some red lines. Theyellow and red lines are convenientlly filtered out, and water-clearplastics having moduli of the kind mentioned easily transmit the bluewith little absorption. High speed photographic negatives areparticularly sensitive to blue so that extremely sharp images can beattained.

A preferred form of the invention will be described below in connectionwith the following drawings, wherein:

FIGURE 1 is a perspective view of certain parts of a conventionalphotoelastic machine incorporating the invention;

FIGURE 2 is an end view of a spark gap device;

FIGURE 3 is a view taken along the lines 3-3 of FIGURE 2; and

FIGURE 4 is a fragmentary view of a preferred form of spark gap.

In FIGURE 1, I have shown a photoelastic machine which for the purposesof clarity has many of the components removed. The machine includes abarrel 1 rotatably mounted on the frame 2. The barrel includes apolarizer 3 and an analyzer 4. The frame 2 has an arm 5 slidablymounting the post 6, which carries the spark gap device 7.

The light from the spark gap is emitted through the aperture 10. Thebeam is directed to a mirror 11 on the post 12 supported by the bracket5. The mirror reflects the beam to a concave mirror 13 which thenprojects the beam through the polarizer and analyzer through thespecimen S, wherein the image is formed and projected to the recordingcamera by conventional lenses and/or mirrors.

The means for imposing an impact load on the specimen includes anupright standard 17 pivotally carrying the arm 14 on which is an impactmember 15. The specimen is supported by having its lower edge rest onthe platform 16 and its right hand edge firm against the inner surfaceof the standard 7. I have found it unnecessary to provide positive meansto lock the specimen in position inasmuch as the transient conditionstake place long prior to the time when forces might cause the specimento move.

The impact is provided by swinging the arm 14 upwardly to the desiredposition and then releasing the same so that it swings down and themember 15 engages the left hand end of the specimen. As will beunderstood by those skilled in the art, the amount of force can becalculated simply by the weight and the distance through which theweight falls to make engagement.

Before going on it might be noted that the barrel 1 is of the type shownin my Patent 2,730,007, and the mirrors 11 and 13 and those mirrors and/or lenses which convey the photoelactic image to the camera are of thetype as shown in my Patent 3,293,908.

The spark gap device 7 will be described in connection with FIGUR-ES 2,3, and 4.

As best shown in FIGURE 2, a plurality of co-axial return, can-typecapacitors 20, 21, 22, 23, 24, and are arranged in a circular patternabout an axis; for example the axis A. Each capacitor has a pair ofelectrical terminals, one of which is the housing and the other a centerpost; for example, for the capacitor 25 the center post 25a and thehousing 25b. The other capacitors have similar electrical terminals.

Within the nest of capacitors and disposed along the axis A are thespark gap electrodes and 31. As best seen in FIGURE 4, the electrode 30is conical in form, the apex 32 of which is coincident with the axis A.The electrode 31 is in the form of a truncated cone, the flat end 33 ofwhich faces the apex 32. The electrode 31 has a passageway 34 made up ofa plurality of bores 35, 36, and 37. By varying the diameter of thehows, the divergent pattern of the passageway can be varied to suit thetype of spark gap. The proportions of the passageway 34 is to allow themirror 13 to be completely filled with light from the gap.

The circuit means interconnecting the capacitor terminals with the sparkgap electrodes is of special significance in that inductance isminimized so that the time constant of the circuit is a practicalminimum. The circuit means also provides for mechanically supportingboth the capacitors and the spark gap electrodes.

Preferably the foregoing takes the form of a pair of spiders 38 and 39,each having six arms which respectively extend radially between thecapacitor terminals and the spark gap electrodes. The arms for thespider 39 are indicated at 40, 41, 42, 43, 44, and 45. The spider 38 hasidentical arms, two of which are indicated in FIGURES 3 at 46 and 47.

Corresponding arms of the two spiders face one another and lie in thesame radial planes. Each arm has an aperture at its outer end; forexample, the apertures 48 and 49 for the arms and 46. The apertures arecoaxial. The head sections of the capacitors extend through theseapertures; for example, the head 25c and post 25a of the capacitor 25.

The two spiders 38 and 39 are spaced from one another along the axis Aby insulating material which preferably comprises a pair of circularsheets of Mylar 50 and 51. The Mylar permits the arms to be spaced veryclosely adjacent one another, which reduces the flux linkage path andthereby minimizes the effect of inductance. The sheets 50 and 51 haveappropriate apertures to accommodate the capacitor heads and thedischarge electrodes. The thickness of the Mylar is determined by whatis needed to insulate the capacitor charging voltage.

The terminals formed by the housings of the capacitors are electricallyconnected to the spider 38, and this is done simply by placing the headsection of each capacitor in abutting relationship with the spider. Forexample, with reference to capacitor 24 in FIGURE 3, it will be seenthat the nut 24d of the head section firmly abuts the arm 46.

The post terminals of the capacitors are electrically connected to theother spider 39. As shown for capacitor 22, the disk 53 is spaced acrossthe aperture 37 and engages the spider 39 and is electrically connectedwith the center post 22a by the nut 54. A similar arrangement is madefor the other capacitors except that the disks for the capacitors 21,23, and 25 are relatively wider as shown on the top of FIGURE 3 for thedisk 55. Also, the latter disks have cavities accommodating connectingnuts such as the cavity 58 accommodating nut 59 on post 25a.

The capacitors 21, 23, and 25 are provided with elongated center postswhich are used for mounting the array of capacitors on the post 6. Asindicated in FIGURES 2 and 3, a triangular shaped bracket 56 made ofinsulating material is supported on the post 6. The center posts 21a,23a, and 25a project through apertures in the bracket and carry nutswhich bear against the bracket and lock the same against the wideneddisks 55, 57 (FIGURE 3), and 58 (FIGURE 1).

The electrode 30 is electrically connected to the spider 38 by athreaded mounting disk 60 secured to the spider by the screws 61. Theelectrode 31 is electrically connected to the spider 39 by a threadedmounting disk 62 fixed to the spider 39 by the screws 63.

The triangular-shaped bracket 56 supports an adaptor 64 mounting a lightcollecting lens 65 and filter 66. The lens 65 collects the light fromthe spark gap coming through the passageway 34 and the filter 66 absorbsthe yellow and red lens so that the beam going to the specimen isessentially monochromatic, i.e., blue.

I apply a capacitor potential between the electrodes 30 and 31 which isof a magnitude that breakdown will not occur in the absence of aninitiating discharge. For initiating the discharge across the gap, I usean annular electrode 70 which surrounds the electrode 30. With theapplication of the voltage between the electrode 70 and the electrode 30as breakdown occurs. This triggers or causes the capacitors todischarge, and illumination energy appears in the gap.

Preferably the electrode 70 is connected to a conductor which isdisposed between the Mylar sheets 50 and 51 and extends radiallyoutwardly so that it can be connected to a high voltage source. Such aterminal is indicated at 71.

From the foregoing description it will be apparent that the spiders 38and 39 not only serve as low level inductance conductors but in additionserve as structural members for supporting the capacitors and spark gapelectrodes. By properly contouring the arms of the spiders, theresistance of each arm can be desirably adjusted to make the circuit L/Rratio of a value to produce a high rate of rise of capacitor dischargecurrent and to make the discharge essentially non-oscillatory.

The circuitry for the electrodes 30 and 31 and the igniter or trigger 71are conventional in form; for example, I have used capacitors of twomicrofarads each charged up to 2,000 volts from a conventional powersupply, and for the igniter I have used a conventional automobile 6-voltignition circuit. Such conventional circuitry is well understood bythose skilled in the art and need not be explained in detail.

One point I want to mention, however, is that the switch to open theignition circuit and cause the voltage to appear across the trigger 71takes the form of a microswitch 72 mounted on the standard 7. Thisswitch is equivalent to a breaker point in the ignition circuit.

The arm (not shown) of the switch 72 is adapted to be contacted by theadjustable screw, 73 secured to the impact device 15. The screw can beadjusted so that the switch is opened at the instant the impact member15 contacts the specimen or opened just slightly after contact. Theswitch 72 and the arm 73 are adjustable for the purpose of correlatingthe time of impact and the discharge across the electrodes 30 and 31 sothat the spark gap discharge or illumination takes place at the exacttime of impact or slightly thereafter. By correlating the illuminationtime with respect to the impact time, the wave can be stopped at finitepoints as it travels along the specimen. Thus a series of images can beobtained which show the actual stress distribution from the time thetraveling wave is generated by impact and as it travels along thespecimen away from the impact point.

I claim:

1. A spark gap device comprising:

a plurality of capacitors arranged in a circular pattern about an axis,and each capacitor having a pair of electrical terminals;

a pair of spark gap electrodes disposed centrally of the capacitors andspaced from one another along said axis, one of the electrodes beingformed with a passageway to conduct light from the spark;

for each pair of said electrical terminals, a pair of closely spacedarms respectively electrically connected with said terminal andextending radially inwardly toward said electrode;

means commonly electrically connecting all of the arms connected withone of the corresponding capacitor terminals to one of the electrodes;

means commonly electrically connecting all of the arms connected withthe other of the corresponding capacitor terminals to the other of theelectrodes;

wherein all of the arms connected to said one electrode are formed froma first unitary piece of conducting, non-magnetic material and all ofthe arms connected to said other electrode are formed from a secondunitary piece of conducting non-magnetic material; and

further including means mounting said capacitors and said one electrodeon said first piece and means mounting said other electrode on saidsecond piece.

2. A construction in accordance with claim 1 further including a triggerelectrode disposed closely adjacent said other electrode.

3. A spark gap device comprising:

a plurality of capacitors arranged in a circular pattern about an axis,and each capacitor having pair of electrical terminals;

a pair of spark gap electrodes disposed centrally of the capacitors andspaced from one another along said axis, one of the electrodes beingformed with a passageway to conduct light from the spark;

a first spider having a central bore coaxial with said axis and having aplurality of arms extending radially outwardly of the bore andterminating respectively adjacent the capacitors;

means for each arm for electrically connecting the arm with one of theterminals of its adjacent capacitor;

means for electrically connecting one of said electrodes with said firstspider;

a second spider having a central bore coaxial with said axis and havinga plurality of arms extending radially outwardly of the bore andterminating respectively adjacent a capacitor, the respective arms ofsaid first and second spiders terminating adjacent a capacitor beingclosely adjacent one another;

for each arm of the second spider means for electrically connecting thearm with the other of the terminals of its adjacent capacitor; and

means for electrically connecting the other of said electrodes with saidsecond spider.

4. A spark gap device comprising:

a pair of fiat, mirror image spiders spaced from one another along anaxis, each spider having a central bore coaxial with said axis and eachspider having a corresponding number of radially extending arms,

and pairs of corresponding arms facing one another, each arm having anaperture adjacent the outer end thereof and the apertures of the pair offacing arms being coaxial;

insulating means in the space between the spiders and each spider havinga central bore coaxial with said axis;

a plurality of capacitors, each capacitor having a housing forming oneof its electrical terminals and a center post forming the otherelectrical terminal, the capacitors being respectively disposed adjacentthe outer ends of a pair of facing arms with the center post of thecapacitor extending through the apertures;

means electrically connecting the housing of a capacitor to itscorresponding arm and means electrically connecting the center post ofthe capacitor to the other of the pair and the arms forming conductivepaths respectively between each housing and said central bore andbetween said center posts and said central bore;

a pair of spark gap electrodes adjacent said bore and spaced from oneanother along said axis, one of the electrodes formed with a passagewayto conduct light from the spark; and

means electrically connecting said one electrode to one of said spidersand means electrically connecting the other of said electrodes to theother of said spiders.

5. A construction in accordance with claim 4, further including meansconnected with said one electrode adjacent the end of said passagewayand mounting a pair of lenses to columnate the light from the spark.

6. A construction in accordance with claim 4, wherein said one electrodeis generally in the form of a truncated cone and the other conical inform with the apex lying along said axis and facing the flat end of saidone electrode.

7. A construction in accordance with claim 5, wherein said insulatingmeans comprises a pair of sheets of Mylar and further including aconductor extending between the sheets and terminating in an annularsection surrounding the other of said electrodes.

References Cited UNITED STATES PATENTS 2,298,114 10/1942 Estorff 315-592,730,007 l/ 1956 Chapman 8814 2,779,890 1/ 1957 Frenkcl 3l3325 X2,911,567 11/1959 Fischer 31559 X 3,300,682 1/1967 Frungel et al. 315-56X 3,361,930 1/1968 Blank 315-59 JOHN W. HUCKERT, Primary Examiner J. R.SHEWMAKER, 1a., Assistant Examiner US. Cl. X.R.

